1. Field of the Invention
This invention relates to gas chromatography. More particularly, this invention relates to gas chromatography employing capillary columns, especially columns of fused silica.
2. Description of the Prior Art
Open tubular columns of capillary dimensions (i.e. less than 0.75 mm ID) are known to be capable of very high resolution. Although various column materials have been used, synthetic fused silica capillary columns have in recent years enjoyed considerable success due to important advantages, including toughness, flexibility and inertness.
The coatings applied to the inner wall of fused silica capillary columns (the so-called liquid phase or stationary phase) are categorized within a broad range extending from non-polar (little or no molecular interaction between the solutes and the liquid phase; that is, separation based on solute vapor pressure differences) to polar (separation based on the molecular interaction between the solutes and the liquid phase).
In order to take full advantage of fused silica capillary tubing, and to extend the range of capillary column applicability, it is necessary to employ fused silica capillary columns coated with polar liquid phases. Non-polar liquid phases offer no significant problems of application. However, the application of polar liquid phases to the inner wall of a fused silica capillary column has presented serious problems. For example, difficulty has been encountered due to the low wettability of a fused silica surface by most polar materials. That is, polar liquid phases tend to bead up on raw fused silica, resulting in an uneven coating with consequent poor performance. The deactivation of the raw fused silica also has presented additional problems.
Various proposals have been made for solving these problems. For example, a variety of surface modification procedures have been developed, including etching and leaching and the addition of different kinds of wetting agents. It now is commonly considered that a pre-coating should be applied to the inner wall of a fused silica column to aid in assuring that the final stationary phase coating can be applied smoothly, and meets other requirements of a chromatographic column. The material of such a pre-coating should be chemically related to that of the final coating in order to achieve good results. For example, if a final coating of cyano and methyl polysiloxane is to be used, a pre-coating of a compound including cyano might be applied first to assure that the final coating will be tightly bound to this pre-coat, evenly distributed, and effective in performance.
The use of such a pre-coating, or a pre-polymer, has not been a fully satisfactory solution to the problems discussed above. For example, satisfactory pre-coating materials are not available for all preferred stationary phases. Also, the pre-coating, when chemically different from the final coating, can interfere with the desired properties of the stationary phase, i.e. the pre-coating can "show through" the final coating and alter the selectivity. Moreover, the application of a pre-coating compound to the column inner wall is a relatively complex procedure, involving a number of closely-controlled steps which add significantly to the cost of producing a column, and make quality control difficult.